A schematic stack structure of a general nitride semiconductor light emitting device and a fabrication method thereof will now be described.
FIG. 1 is a sectional view of a general nitride semiconductor light emitting device.
Referring to FIG. 1, a conventional nitride semiconductor light emitting device includes a substrate 101, a buffer layer 103, an n-GaN layer 105, an active layer 107 and a p-GaN layer 109.
In detail, in order to minimize the occurrence of crystal defects due to differences in the lattice constants and the thermal expansion coefficients of the substrate 101, for example, a sapphire substrate, and the n-GaN layer 105, the buffer layer 103 is formed of a GaN-based nitride or an AlN-based nitride having an amorphous phase at a low temperature. After the buffer layer 103 is formed, the n-GaN layer 105 doped with silicon at a doping concentration of 1018/cm3 is formed at a high temperature as a first electrode contact layer. Thereafter, the growth temperature is lowered and the active layer 107 is formed. Thereafter, the growth temperature is again elevated and the p-GaN layer 109 doped with magnesium (Mg) is formed.
The nitride semiconductor light emitting device having the aforementioned stack structure is formed in a p-/n-junction structure which uses the n-GaN layer 105 as the first electrode contact layer and uses the p-GaN layer 109 as the second electrode contact layer.
A second electrode material formed on the second electrode contact layer is limited depending on a doping type of the second electrode contact layer. For example, in order to decrease the contact resistance between the second contact material and the p-GaN layer 109 having a high resistance component and enhance the current spreading, a thin transmissive resistance material of a Ni/Au alloy is used as the second electrode material. The n-GaN layer 105 used as the first electrode contact layer is made in a single crystal structure which has a higher growth temperature than and is thicker than the buffer layer 103 having the amorphous crystallinity. In particular, in the conventional p-/n- junction light emitting device structure, the n-GaN layer 105 used as the first electrode contact layer is doped with Si of 1018/cm3-1019/cm3 and is grown in a thickness within 5 μm which is thicker than the polycrystal GaN or AlN buffer layer 103 including a low temperature amorphous crystallinity, thereby controlling the crystallinity.
The n-GaN layer 105 used as the first electrode contact layer has the characterizations of electrical conductivity, a resistance linearly decreased as the silicon doping concentration increases, and an operation voltage decreased when a forward bias is applied. On the contrary, when a reverse bias is applied, a breakdown voltage (Vbr) is decreased, so that a leakage current is increased, and as time elapses, the optical power is decreased, which badly influences the life time of the light emitting device.
Also, in adjusting the doping concentration, when the doping concentration is more than 1019/cm3 (i.e., heavily doped), an extreme strain is generated to break the n-GaN layer 105. As the silicon doping concentration increases, point defects on a surface of the n-GaN layer 105 increase. Therefore, the increased point defects badly influences the active layer 107 emitting light, so that they function as a current pathway when a forward or a reverse bias is applied and function as a factor increasing the leakage current. The conventional light emitting device having the aforementioned drawbacks is subject to a limitation in the reliability.
In addition, the conventional nitride semiconductor light emitting device includes crystal defects, such as ‘threading dislocation’, ‘screw dislocation’, ‘line dislocation’, ‘point defect’, or ‘mixture’. Especially, the ‘threading dislocation’ is propagated in the sapphire substrate to the surface of the light emitting device. During the propagation, the ‘threading dislocation’ passes through the active layer emitting light. Therefore, the ‘threading dislocation’ later serves as a current path for leakage current, and when a high voltage such as ESD or the like is instantly applied, the active layer is destroyed or the optical power is lowered. The above problems provide fundamental reasons badly influencing the device reliability.